Biology 3A Lab: Mendelian, Human & Population Genetics (03/09) Page 1 of 11 Biology 3A Laboratory Mendelian, Human and Population Genetics OBJECTIVE • To be able to follow alleles through meiosis and determine gametes. • To study the principles of inheritance. • To understand the basic types of inheritance patterns in humans and other multicellular organisms. • To solve genetic problems involving monohybrid, dihybrid and trihybrid crosses. • To solve genetic problems when the genotype of the parents or offspring are known and unknown. • To observe and record some phenotypes which occur in humans. • Calculate Hardy-Weinberg equation. INTRODUCTION Genetics is the branch of biology that examines the inheritance of traits and how these traits are passed from one generation to the next. Much of what we know about genetics today stems from Gregor Johann Mendel’s studies of pea plants during the 1860’s. Mendel has been credited with the founding of genetics. The significance of his investigations, however, were not recognized until well after his death. Through his work with pea plants, Mendel was able to observe and track individual characteristics and employ simple mathematical models in predicting the outcomes of his investigations. His conclusions can be summarized as four “laws” of heredity: 1. Law of Unit Character: characteristics are inherited as separate units. 2. Law of Dominance: for some characteristics, the presence of one gene (the dominant gene) will mask the presence of another gene (the recessive gene), resulting in the expression of the dominant gene. 3. Law of Segregation: all cells in the body contain a pair of characteristics (genes). During gamete formation in the reproductive cells, each gamete will only receive one member of each pair. 4. Law of Independent Assortment: during gamete formation, a pair of genes for a particular characteristic will be inherited independently of other genes that code for any other characteristic(s). Although Mendel’s experiments with pea plants paved the way for understanding the mechanisms involved in heredity, it wasn’t until the discovery of chromosomes and the meiotic process that inheritance become more clearly understood. If you recall, all diploid (2n) cells in your body contain a complete (two) set(s) of homologous chromosomes. This complete set of chromosomes is composed of a maternal and paternal pair. Each pair of homologous chromosomes contains genes that determine specific traits by coding for the production of a specific protein. Each gene also has a specific location (locus) on a specificBiology 3A Lab: Mendelian, Human & Population Genetics (03/09) Page 2 of 11 chromosome and may have several versions/forms called alleles. For example, in pea plants, height is governed by a single gene which can have two versions, T and t. Every diploid cell has two copies of one gene which make up the homologous pair of chromosomes that determine a particular trait. These two alleles could be either the same (homozygous) or different (heterozygous). In either case, these alleles together will determine an organism’s genetic make-up (genotype). A. UNDERSTANDING MEIOSIS & CHROMOSOME SEGREGATION After meiosis (sexual reproduction), the genetic traits of an organism are segregated and readied to be passed from parent to offspring. When sexually reproducing organisms undergo meiosis, they produce gametes that are haploid (n). Haploid cells include sperm and ovum (unfertilized egg) that have half the number of chromosomes as the original precursor cell. When humans undergo meiosis, this means that the sperm or ovum will contain 23 chromosomes as opposed to the 46 chromosomes found in diploid cells. So, in order to maintain a constant number of chromosomes in successive generations, a reduction in chromosome number between successive fertilizations (fusion of the male and female gametic nuclei) is necessary and is accomplished through meiosis. Recall from the lecture material that meiosis in the female is called oogenesis and in the male, spermatogenesis. In this exercise, you will review your understanding of the meiotic process by diagramming the separation of chromosomes in an organism with a diploid number of 4. Procedure: 1. Prior to drawing and labeling the meiosis diagram in your worksheet, practice/go over the meiotic process. If you need assistance, please ask your laboratory instructor. 2. Make sure that you understand the following terms before you begin: chromatin, chromosome, chromatid (sister chromatids) and centromere. 3. Assume a diploid number of 4 for this organism. This means that this organism will have inherited 2 chromosomes from the mother and 2 chromosomes from the father. Let the red colored chromosomes represent the maternal set and the yellow colored chromosomes represent the paternal set. 4. Here’s what you’ll have: one long yellow chromosome, one long red chromosome, one short yellow chromosome and one short red chromosome. This will give you four chromosomes (2n = 4). 5. Place the four chromosomes on the meiosis board in the large cell located at the top of the board. This cell is in interphase. Using the four remaining chromosomes to the side of the board, demonstrate DNA replication (synthesis). 6. You should now have four duplicated chromosomes (each chromosome should now be composed of two sister chromatids attached at the centromere) in the large cell. 7. Using the duplicated chromosomes, complete meiosis I and then meiosis II. 8. Have your instructor sign off after you have demonstrated the meiotic process. B. Importance of sample size in probability Before we begin to predict the possible outcomes of genetic crosses, you must first understand probability, which is the chance of an event occurring out of the total number of possible events. For example, if you flip a coin, there is a one in two chance (50%) of the coin coming up heads for a two sided coin. Another example that you may be familiar with is the rolling a die. There is only one chance for rolling a “six” on a six-sided die. Often